Texas Instruments TPS3801, Отчет о приложении

Страница: 14 / 28

Voltage Supervision at the V
DD
Terminal
10 SLVA075
3.4 Making the Supervisor Circuit Less Sensitive
Supply voltage supervisors are designed to detect small voltage drops below the
threshold voltage. The datasheet of TPS380x specifies, that a reset pulse from
200 mV above the threshold voltage V
IT
to 200 mV below V
IT
with a duration of
3
µ
s suffices to trigger a reset pulse. But not every system requires the detection
of such small spikes, because these spikes do not influence the functionality of
the system. In this case, it is possible to reduce the sensitivity of the undervoltage
recognition.
A simple solution to this problem is to add a low pass filter in front of the V
DD
input.
The recommended capacitor at V
DD
works as a part of the RC network.
VDD RESET
GND
TPS3801J25
C = 100 nF
R
Figure 11. TPS3801 With a Low Pass Filter at V
DD
Terminal
Care has to be so taken that the additional resistor does not alter the threshold
voltage too much. The threshold voltage is determined by the internal voltage
divider, like shown in the block diagram in Chapter 2. A resistor of R
1
= 10 k
Ω
in
the RC combination raises the threshold voltage by 1.5% independent of the
voltage version of the device. A R
=1 k
Ω
resistor causes a rise in the threshold
voltage of 0.15%, so that it can be ignored.
Figure 12 shows a measurement with different RC networks. The value of the
capacitor is C = 100 nF, the resistor values are 100
Ω
, 1 k
Ω,
and 10 k
Ω
. The device
is a TPS3801J25 with a threshold voltage of 2.25 V
.
0.00
0.50
1.00
1.50
2.00
2.50
1.0 10.0 100.0 1000.0
V
oltage – V
30.0 9.0 1.5
t – Time –
µ
s
100
Ω
1 k
Ω
10 k
Ω
C = 100 nF = constant
VIT = 2.25
Figure 12. Breakdown Voltage versus Pulsewidth With a Low Pass Filter